Diblocked diisocyanate diurea oligomers and coating compositions comprising same

ABSTRACT

This invention provides novel resin systems and high solids, solvent-based chain-extendable, self-crosslinking coating compositions comprising same. The resin system comprises novel diblocked diisocyanate diurea oligomers of number average molecular weight about 300 to 5000. The resin system further comprises a polyepoxide, preferably a diepoxide, of molecular weight about 100 to 1000 used generally with said oligomer in weight ratio of about 1:1 to about 1:10, respectively. The resin components provide chain-extension polymerization during cure at elevated temperature, in situ, on the surface of a substrate. The resin system is also self-crosslinking. That is, no additional crosslinking component is required to cure the composition. The cured coatings of the invention provide greatly improved physical properties, in particular, greatly improved corrosion resistance.

RELATED APPLICATIONS

This is a division of application Ser. No. 334,842, filed Dec. 28, 1981,now U.S. Pat. No. 4,396,753.

This application is related to U.S. Pat. No. 4,409,381, entitled NovelDiblocked diisocyanate Urea Urethane Oligomers and Coating CompositionsComprising Same, U.S. Pat. No. 4,409,380, entitled Novel TertiaryAlcohol-Diblocked Diisocyanate Urea Urethane Oligomers and CoatingComposition Comprising Same, and application Ser. No. 334,792, filedDec. 28, 1981 entitled Novel Tertiary Alcohol-Diblocked DiisocyanateDiurea Oligomers and Coating Compositions Comprising Same.

INTRODUCTION

This invention relates to novel resin systems and to high solids,solvent based coating compositions comprising same. A first componentcomprises any of certain novel chain-extendable, crosslinkable diblockeddiisocyanate diurea oligomers. The second comprises polyepoxide. Theresin components provide chain-extension polymerization during cure atelevated temperature, in situ, on the surface of a substrate. The resinsystem is self-crosslinking, that is, no additional crosslinking agentis required. The cured coatings of the invention are highly humidity andsolvent resistant and provide exceptional corrosion resistance.

BACKGROUND OF THE INVENTION

Solvent based coating compositions are known which employ high molecularweight (e.g. 2,000 to 10,000) polymer resins having crosslinkingfunctionality, and a suitable crosslinking agent. Typically, suchcoating compositions are applied to a substrate, for example, byspraying, and are then cured by baking the coated substrate at anelevated temperature suitable to drive off the organic solvent and topromote the crosslinking reaction. The resulting thermoset coating, ifsufficiently humidity, solvent and corrosion resistant, can provideaesthetic and functional advantages including corrosion protection forthe underlying substrate.

Coating compositions comprising such high molecular weight polymerresins typically comprise only 25% to 50% solids so as to be sprayableor otherwise conveniently applicable to a substrate. The viscosity ofcoating compositions of higher solids content is typically too high forthis purpose. Conventional epoxy ester based automotive vehicle sprayprimers, for example, typically have a volatile organic content ("VOC")of approximately 540 g/l.

Elimination of the volatile organic solvent portion during curing ofthese conventional low-solids coating compositions presents toxicity andin some cases flammability hazards. Furthermore, bulk volume of thesecoating compositions is relatively large and therefore presentsundesirable material handling difficulties, and added expense.Furthermore, excessive solvent losses and/or solvent recovery equipmentadd considerable expense to the coating operation. Recently,governmental regulations on hydocarbon emissions, particularlyapplicable to automotive coating operations, mandate a significantreduction in volatile organic content for coating compositions. Thus,for example, governmental guidelines for 1982 presently require thatemissions of volatile organics from automotive vehicle primer coatingoperations be reduced to that equivalent to using coating compositionsof no greater than 350 g/l (2.9. lb./gal.) VOC. To meet that guideline,coating compositions of VOC greater than 350 g/l can be employed inconjunction with emissions treatment equipment to achieve the specifiedemissions limit. Such treatment equipment presents significantadditional expense, however, and thus there is a great need to providecoating compositions of VOC reduced near to, or preferably even lowerthan, the 350 g/l governmental limit.

In response to these concerns, high solids coating compositions havebeen suggested which, typically, employ low molecular weightmulti-functional adducts or copolymers in combination withmulti-functional crosslinking agents. These high solids coatingcompositions are less viscous and, therefore, can be applied byspraying, for example, with far lower VOC than was possible withconventional epoxy ester based coating compositions or otherconventional coating compositions comprising high molecular weightpolymer resins. After application to the substrate, high solids coatingcompositions are cured by baking at a cure temperature, that is, at anelevated temperature suitable to drive off the volatile organic contentand to promote polymerization and crosslinking of the multi-functionallow molecular weight component(s).

Typically, high solids coating compositions yield cured coatings havingpolymeric networks that differ significantly in structure and morphologyfrom the polymeric networks provided by conventional, low solids coatingcompositions comprising high molecular weight polymers. Consequently,the physical properties of the coatings provided by such high solidscoatings compositions can differ significantly from those of the curedcoatings provided by the conventional, low solids coating compositions.In particular, the cured coatings obtained from known high solidscoating compositions can be inferior in that they can be less flexible,less solvent resistant, less adherent to the substrate and/or for otherreasons provide less corrosion inhibition for the underlying substrates.Accordingly, it would be highly desirable to provide a coatingcomposition comprising low molecular weight materials suitable for usein high solids, solvent based coating compositions and yet which, uponcuring, form coatings having physical properties, particularly corrosionresistance, comparable to or better than the physical properties ofcoatings obtained from conventional low solids solvent based coatingcompositions.

Accordingly, it is an object of the present invention to provide novelresin compositions suitable for use in high solids, solvent-basedcoating compositions. In this regard, it is a particular object of theinvention to provide novel coating compositions which are curable bychain-extension and crosslinking during cure, in situ, on the surface ofa substrate to form polymeric coatings similar in properties to thoseobtainable through use of conventional low solids, solvent-based coatingcompositions.

It is a particular object of the invention to provide a coatingcomposition of sufficiently low VOC to facilitate compliance withgovernmental guidelines. It is also an object of the invention toprovide such a coating composition which can be applied to a substrateby spraying or other known method.

It is another object of the invention to provide a method of making acoating on a substrate, which coating has advantageous physicalproperties including humidity, solvent and corrosion resistance.Additional aspects and advantages of the invention will be apparent fromthe following description thereof.

SUMMARY OF THE INVENTION

According to the present invention, low molecular weightchain-extendable, crosslinkable diblocked diisocyanate diurea oligomersare provided which are suitable for use in high solids, organic solventbased coating compositions. The novel oligomers of the invention are thereaction product of half-blocked diisocyanate with suitable diamine andhave a number average molecular weight of preferably about 300 to about5000, more preferably about 300 to about 1500. The half-blockeddiisocyanate is the reaction product of organic diisocyanate withmonofunctional blocking agent.

According to another aspect of the invention, a novel solvent-basedresin composition comprises the novel chain-extendable, crosslinkablediblocked diisocyanate diurea oligomer of the invention, polyepoxidebearing preferably about 2 to 10, more preferably about 2 to 4 epoxidegroups, and having molecular weight of about 100 to 1000, morepreferably about 300 to 700, and suitable organic solvent. The resincomposition comprises latent chain-extension polymerizationfunctionality and undergoes both chain-extension polymerization andcrosslinking reaction, in situ, on the surface of the substrate duringcure of the coating to form high molecular weight polymers. Morespecifically, the blocked isocyanate groups of the novel oligomers ofthe invention undergo thermal de-blocking at elevated temperaturebetween about 120° C. and 250° C., in more preferred embodiments betweenabout 150° C. and 220° C., and then react with the epoxy functionalityof the polyepoxide forming a polymer bearing oxazolidone linkages. Theresin composition further comprises latent self-crosslinkingfunctionality. That is, the de-blocked isocyanate functionality is alsoreactive with the N-hydrogen functionality of the urea moieties of thechain-extended polymerization product which is formed during cure of thecoating composition. That is, during cure of the coating composition, insitu, on the surface of the substrate, each blocked isocyanate group isde-blocked and will undergo chain-extension reaction with an epoxy groupof the polyepoxide component of the coating composition or will undergocrosslinking reaction with a urea moiety of the chain-extendedpolymerization reaction product forming during cure. If the polyepoxideis a diepoxide, then the epoxy isocyanate reactions will providesubstantially linear chain-extension polymerization. If the polyepoxidebears three or more epoxide groups, then epoxy/isocyanate reaction willprovide not only chain-extension polymerization but also an additionalmode of crosslinking in the cured coating. Thus, the degree ofcrosslinking in the cured coating can be controlled, in part, by theselection of the polyepoxide employed in the coating composition.Accordingly, the novel resin composition of the invention is curable toform a coating, in situ, on the surface of a substrate, employing both achain-extension polymerization reaction and separate and distinctcrosslinking reactions.

The resin composition of the invention can be formulated into highsolids coating compositions having a viscosity as low as about 25-40seconds, #4 Ford Cup or less, at 27° C. at calculated VOC of 350 to 400g/l or less.

According to another aspect of the invention, a method of making acorrosion, solvent and humidity resistant coating on a substratecomprises applying to the substrate the novel resin composition of theinvention and heating the resin composition to between about 120° C. andabout 250° C. and preferably to between about 150° C. and about 220° C.for a period sufficient to yield a cured coating. The cured coating,which is yet another aspect of the invention, is solvent and humidityresistant and has been found to provide exceptionally good corrosionresistance.

DETAILED DESCRIPTION OF THE INVENTION

As used herein, a high solids coating composition is one comprisingpolymerizable resin in which a volatile organic solvent content of about400 g/l (3.4 lb./gal.) or less yields a viscosity of less thanapproximately 40 sec. #4 Ford Cup at 27° C. (80° F.). Thus, such highsolids coating composition could be applied, for example, by spraytechniques to a substrate.

The novel crosslinkable, chain-extendable diblocked diisocyanate diureaoligomer of the invention preferably has a number average molecularweight about 300 to about 5000, more preferably about 300 to about 1500.These oligomers are provided as the reaction product of a suitablediamine with suitable half-blocked diisocyanate. The two amine groups ofthe diamine each react with the free isocyanate functionality of adifferent half-blocked diisocyanate molecule, each forming a urealinkage. The reaction product comprises two blocked isocyanate groups,one from each of the two half-blocked diisocyanate molecules whichreacted with the diamine. The diamine and half-blocked diisocyanate arereacted together according to methods which are well known to theskilled of the art. It will be within the skill of the art in view ofthe present disclosure to select a suitable diamine, of which many,including aromatic and aliphatic diamines, are readily commerciallyavailable. Preferred aliphatic diamines are of molecular weight about 50to about 700, more preferably about 50 to about 300 and include, forexample, alkylenediamines, wherein the alkylene moiety is straight orbranched chain and has, preferably about from 2 to about 20, morepreferably about 3 to about 12 carbons, and of which terminalalkylenediamines, that is, alkylenediamines bearing two terminal aminefunctionality, are most preferred, for example, 1,6-hexanediamine,1,5-pentanediamine, 1,4-butanediamine and a mixture of any of them. Alsopreferred are cycloaliphatic diamines of about 4 to about 20 carbons,for example, isophorone diamine, and aromatic diamines wherein eachamine is substituted on the same benzene ring or on different benzenerings linked through a covalent bond or through one or more carbons of aone to seven carbon aliphatic moiety, for example, toluene diamine and4,4'-methylenedianiline.

Lower molecular weight aliphatic diamines, those of one to six carbons,for example, ethylenediamine and hexanediamine, are preferred overaromatic diamines if providing a very high solids content (that is, lowVOC) coating composition is of primary concern. The lowest molecularweight aromatic diamines, for example, toluene diamine, have seven ormore carbons and require a greater VOC for any desired viscosity.

Suitable half-blocked diisocyanates comprise the reaction product of asuitable organic diisocyanate in approximately 1:1 molar ratio withsuitable monofunctional blocking agent, and include those of numberaverage molecular weight about 120 to about 2000, preferably about 120to about 600. Suitable organic diisocyanates are readily commerciallyavailable and will be apparent to the skilled of the art in view of thepresent disclosure. Suitable diisocyanates include aromaticdiisocyanates, for example, phenylene diisocyanates and toluenediisocyanates, and aliphatic diisocyanates, for example, isophoronediisocyanates and diisocyanatoalkane wherein the alkyl moiety haspreferably from about three to ten carbons, for example,1,4-diisocyanatobutane and 1,6-diisocyanatohexane and the like or acompatible mixture of any of them. Most preferably, the organicdiisocyanate has molecular weight less than about 250. If corrosionresistance is of primary concern in the cured coating, for example, inthe case of an automotive vehicle primer or topcoat, it may be preferredto use an aliphatic diisocyanate, for example, isophorone diisocyanateand 1,1-hexane diisocyanate. Aromatic diisocyanates provide suitablecoatings, however, and may be preferred in view of their lower cost.

Suitable half-blocked diisocyanate is prepared by reaction of anysuitable organic diisocyanate, as described above, with sufficientmonofunctional blocking agent to block approximately one half of theisocyanate functionality. Accordingly, monofunctional blocking agent isreacted with organic diisocyanate in approximately 1:1 molar equivalentratio. Suitable techniques well known to the skilled of the art can beemployed to maximize the yield of half-blocked diisocyanate, such as,for example, adding the blocking agent slowly to the organicdiisocyanate under reaction conditions. The half-blocked diisocyanate isthen reacted with the previously described diamine in molar ratio ofabout 2:1, respectively, to produce the chain-extendable, diblockeddiisocyanate diurea oligomer of the invention.

Suitable readily commercially available monofunctional blocking agentsare well known to the skilled of the art. The blocking agent is selectedsuch that the blocked isocyanate group will remain blocked for longperiods of time at normal storage temperatures, but will besubstantially totally "de-blocked" at elevated "cure" temperature. Inaddition, since the blocking agent will be released when the coatingcomposition is cured by baking, it is preferred that the blocking agenthave high volatility near its de-blocking temperature and so willdiffuse rapidly through the coating composition and evaporate completelytherefrom during the baking step. Any blocking agent allowed to remainin the cured coating should be inert to the cured coating and to thesubstrate and to any other coatings to be used in conjunction with it.It is within the skill of those skilled in the art to select a suitableblocking agent to provide a de-blocking temperature meeting therequirements of each particular application intended for a coatingcomposition of the invention. It will typically be preferred that theblocked isocyanate functionality be de-blocked (i.e., that the coatingcomposition be curable) at a temperature within the range of about 150°to 220° C. Accordingly, preferred monofunctional blocking agents areselected from the group comprising suitable amides, for examplecaprolactam, phenols, ketoximes and lower alcohols, for example, primaryand secondary alkanol of one to about eight carbons, for examplemethanol, ethanol, any propanol, any butanol, any pentanol, includingcyclopentanol, and the like, or a mixture of any of them.

The novel solvent based resin compositions of the invention comprise thenovel chain-extendable crosslinkable diblocked diisocyanate diureaoligomers of the invention and suitable polyepoxide. Preferredpolyepoxides have from 2 to about 10 epoxide functionality, morepreferably from 2 to about 4. Preferably the polyepoxide (or each ofthem) has a number average molecular weight between about 100 and 1000,and more preferably between about 300 and 700. Numerous suitablepolyepoxides are readily commercially available and will be apparent tothe skilled of the art in view of the present disclosure. Preferredpolyepoxides include, for example, any of a wide variety of acyclic orcyclic aliphatic polyepoxides such as, for example, 1,4-butane dioldiglycidyl ether, vinylcyclohexene diepoxide and Araldite Cy179(trademark, Ciba-Geigy Corporation, Ardsley, N.Y.), and aromaticpolyepoxides such as, for example, Bisphenol A epichlorohydrin epoxyresins and the like or a compatible mixture of any of them. Preferredpolyepoxides include terminal polyepoxides, that is, polyepoxidesbearing two terminal epoxide groups, since these are generally morereactive and therefore provide coating compositions which cure fasterand/or at lower temperature. Preferred polyepoxides include diepoxides.Most preferred in view of their commercial availability are, forexample, for example, acyclic or cyclic aliphatic diepoxides such as,for example, 1,4-butanediol diglycidyl ether and 4-vinylcyclohexenedioxide and aromatic diepoxides such as, for example, Bisphenol Aepichlorohydrin epoxy resins, for example, Epon 828 (trademark) andother members of the Epon (trademark) series, Shell Chemical Company,Houston, Tex., and DER 331 (trademark) and other members of the DER(trademark) series, Dow Chemical Company, Midland, Mich. Also preferredare cycloaliphatic diepoxy resins, for example, the Eponex (trademark)series of Shell Chemical Company, Houston, Tex., and hydantoin epoxyresins, for example, Resin XB2793 (trademark), Ciba-Geigy Corporation,Ardsley, N.Y. and epoxy novalak resins, for example, Epon 152(trademark) and Epon 154 (trademark), Shell Chemical Company.

In general, of the above described diepoxides, the lower molecularweight diepoxides are preferred, for example, Epon 828 (trademark),since a resin composition of correspondingly lower viscosity (or lowerVOC) is provided. Other, higher molecular weight diepoxides, forexample, higher molecular weight members of the Epon (trademark) series,are suitable for coating compositions of somewhat higher viscosity (orlower solids content). The higher molecular weight members of the Eponseries, for example Epon 1001 and Epon 1004, and the like may besomewhat less preferred, since they afford coating compositions ofhigher VOC (or lower solids content) and with de-blocked isocyanatefunctionality. Also, however, improved properties, for example, improvedcorrosion resistance have been achieved with coating compositionscomprising polyols prepared using such materials and the choice ofsuitable polyepoxide will depend upon the particular applicationintended for the coating composition.

Polyepoxide can be used in approximately stoichiometric amount in thecoating composition. That is, the coating composition can comprise equalmolar equivalents of polyepoxide and diblocked diisocyanate diureaoligomer. Such compositions provide efficient chain-extensionpolymerization during cure of the coating composition. While there canbe an excess of oligomer, there should not be an excess of polyepoxide,since unreacted epoxy functionality can adversely effect the humidity,solvent and corrosion resistance of the cured coating. Preferably theoligomer and polyepoxide are used in a weight ratio of 1:1 to about 10:1equivalent weights, respectively, and more preferably in a ratio of 1:1to about 2:1.

While not wishing to be bound by theory, it is presently understoodthat, upon curing the coating composition, the polyepoxide and thede-blocked diisocyanate diurea oligomer undergo chain-extension reactionto form a polymer bearing oxazolidone ring linking moieties. Thepolyoxazolidone so formed is believed to crosslink through reaction ofN-H functionality of the urea moieties of the polymer with de-blockedisocyanate functionality of the oligomers to form ureylene linkages. Thedegree of crosslinking in the cured coating provided through this modeof crosslinking can be controlled by selection of the ratio of epoxidefunctionality to isocyanate functionality in the resin composition. Thegreater the excess of isocyanate functionality, the higher the crosslinkdensity in the cured coating.

In addition to crosslinking by de-blocked isocyanate/urea N-hydrogenreaction, the polyepoxide can provide crosslinking in the cured coating.While a diepoxide which has undergone chain-extension reaction with ade-blocked isocyanate functionality of each of two oligomer moleculesprovides no additional epoxy functionality, a polyepoxide of three ormore epoxide groups which has undergone chain-extension reaction withtwo de-blocked oligomer molecules can react with the de-blockedisocyanate of one or more additional oligomers and thereby provide anadditional mode of crosslinking and thus a more highly crosslinkedpolymeric structure in the cured coating. Accordingly, in general,diepoxides are preferred over polyepoxides of 3 or more epoxide groupsif greater flexibility is desired in the cured coating. It should berecognized, however, that higher homologs of the epoxy resins useful inthe present invention often comprise hydroxy functionality in additionto the epoxy functionality. It is believed that de-blocked isocyanatefuncitionality of the oligomer will react with such hydroxyfunctionality. This aspect of the invention provides significantadvantage where more efficient crosslinking is desired.

It will be appreciated from the above, that network crosslink densitycan be controlled, and therefore the flexibility and other properties ofthe cured coating can to a large extent be controlled by selection ofthe polyepoxide and of the diblocked diisocyanate diurea oligomer usedin the coating composition. Crosslink density increases and flexibilitydecreases as the epoxide equivalent weight of the polyepoxide is reducedand/or as the isocyanate equivalent weight of the diblocked diisocyanatediurea oligomer is reduced. Thus, it will be apparent to the skilled ofthe art in view of the present disclosure that the selection of thepolyepoxide and the selection of the organic diisocyanate, blockingagent and alkanolamine reactants for preparing the diblockeddiisocyanate diurea oligomer provides substantial control of thecrosslink density in the cured coating. Thus, for example, if thepolyepoxide used is diepoxide, there will be a lower crosslink densityin the cured coating than if the polyepoxide used is of similarmolecular weight but has three or more epoxy groups per molecule.

Sufficient solvent is used in the coating composition of the inventionto reduce the viscosity of the coating composition to a level suitablefor application to the substrate in the desired manner. The molecularweight of the polyepoxide and of the diblocked diisocyanate diureaoligomer will affect the volatile organic content of the coatingcomposition at a desired viscosity. Where a high-solids coatingcomposition is desired, preferably lower molecular weight components areemployed, since this has been found to provide high-solids coatingcompositions which can be applied easily to a substrate by spray orother means in a coating composition having a calculated volatileorganic content of about 350 g/l (2.9 lb./gal.) or less. Morespecifically, while conventional epoxy ester-type automotivespray-applied primer coating compositions are known to require avolatile organic content of about 540 g/l, the novel coatingcompositions of the present invention require as little as about 350 g/lto 400 g/l (2.9 lb./gal. to 3.4 lb./gal.) or less VOC (calculated) toprovide a viscosity of less than about 40 sec., #4 Ford Cup at 27° C.(80° F.). Of course, the coating compositions of the invention need notbe formulated as a "high solids" compositions, but rather can have ahigher VOC to provide a lower viscosity. It is generally preferred inautomotive vehicle spray coating applications and the like, for example,that sufficient solvent be used to provide a viscosity of about 15 to 40sec., #4 Ford Cup at 27° C. (80° F.).

It will be within the skill of the art to determine the proper volatileorganic content for a given coating composition of the invention for agiven application. In general, suitable solvents include, for example,Cellosolve (trademark), Butyl Cellosolve (trademark), Butyl CellosolveAcetate (trademark), Hexyl Cellosolve (trademark), Hexyl CellosolveAcetate (trademark), Proposol P (trademark), Proposol B (trademark),Proposol M (trademark), all of Union Carbide Corporation, New York,N.Y., butanol, methyl ethyl ketone, methyl amyl ketone and the like, ora compatible mixture of any of them. Additional suitable solvents willbe apparent to the skilled of the art in view of the present disclosure.

Any solvent allowed to remain in the cured coating should be inert so asto avoid adverse effect upon the cured coating or upon another coatingused in conjunction with it during the curing process or thereafter.Preferably the cured coating is completely free of solvent. Thepreferred solvents, in addition, have relatively low volatility attemperatures appreciably below their boiling point such that solventevaporation is low during storage and/or application of the coatingcomposition to the substrate.

Also preferably included in the coating composition of the invention isany of a variety of commercially available catalysts which, in view ofthe present disclosure, will be apparent to the skilled of the art to besuitable to catalyze the chain-extension polymerization reaction betweenepoxide and de-blocked isocyanate functionality. Suitable catalystsinclude, for example, alkoxy bases, for example, sodium phenoxide,sodium methoxide, sodium butoxide, and tertiary amines, for example,hexamethylene triamine, and the like or a compatible mixture of any ofthem. Some catalyst may however reduce the stability of the coatingcomposition. In addition, any of a variety of catalysts for theisocyanate de-blocking reaction can also be included in the coatingcomposition, for example, dibutyl tin dilaurate. Also suitable ascatalyst are Anchor 1040 (trademark) and Anchor 1115 (trademark)available from PVO International Corporation, San Francisco, Calif.,which comprise amine blocked boron trifluoride and which have been foundto catalyse the isocyanate de-blocking reaction. However, it should berecognized that amine catalyst should generally not be employed togetherwith acid catalysts.

In addition, flow control agent, for example, polybutyl acrylate;wetting agent, for example, silicone; pigments; pigment dispersents,corrosion inhibitors, for example, chromate pigments, numerous of all ofwhich are known to the skilled of the art, may be employed in thecoating compositions of the invention.

According to another aspect of the invention, a coating on a substrateis provided, which coating comprises the chain-extended, crosslinkedpolymer product following cure of a coating comprising the resin systemof the invention. The coating composition can be a low solidscomposition, that is, it can have a high VOC, but generally a highsolids composition, that is, one having a low VOC, is preferred for thereasons given above. It can be applied by any conventional method,including brushing, dipping, flow coating, spraying, etc. Spraying willgenerally be preferred, for example, for applying the composition as anautomotive vehicle body primer or topcoat. In such sprayingapplications, the coating compositions of the invention are especiallyadvantageous for use as high solids compositions.

Curing the coating composition requires baking for sufficient time atsufficiently elevated temperature to de-block the blocked isocyanatefunctionality of the diblocked diisocyanate diurea oligomers and topromote the subsequent chain-extension and crosslinking reactions. Thetime and temperature required to cure the coating are interrelated anddepend, in part, upon the particular diblocked diisocyanate diureaoligomer, polyepoxide, sont and other materials, if any, used in thecoating composition and upon the relative proportion of each. Employinga volatile organic content of about 360 g/l and selecting preferredcomponents as described above, the required bake time and temperature istypically about 20 to 30 minutes at about 180° C. The temperaturerequired for cure can be reduced to about 150° C. for 20 to 30 minutesby addition of suitable catalyst such as any of those described above orotherwise known to the skilled of the art, for example, dibutyl tindilaurate and tertiary amine.

It is a significant advantage of the coating composition of theinvention that it does not require the addition of crosslinking agent.Known high solids coating compositions employing melamine crosslinkingagent, for example, are disadvantaged by the cost of same and arefurther disadvantaged by the problems associated with the possibleevolution of formaldehyde during cure of such coating compositions. Itis another advantage of the invention, notwithstanding that acrosslinking agent is not added to the coating composition of theinvention, that the crosslink density can be easily controlled to alarge extent. As indicated above, the molecular weight and carbon chainlengths of the polyepoxide and of the reactants for preparation of thediblocked diisocyanate diurea oligomers of the invention can be selectedto provide the desired crosslink density in the cured coating. Inaddition, the number of epoxy groups per polyepoxide molecule affordscontrol of the crosslink density in the cured coatings. In addition, therelative proportion of polyepoxide to oligomer can be selected tocontrol crosslink density, since excess oligomer can providecrosslinking via the active hydrogen of the urea moieties of the polymerproduct of the chain-extension reaction.

High solids coating compositions according to the present invention,have been found to afford cured coatings with good humidity and solventresistance and with exceptionally good corrosion resistance. Thecorrosion resistance has been found to be comparable and even betterthan that of conventional epoxy ester based, low solids sprayablecoating compositions. The significant reduction in volatile organiccontent provided by the high solids coating composition of the inventioncan be seen to present, therefore, a highly advantageous advance in theart. Thus, for example, cured coatings according to the invention havebeen found to provide excellent corrosion resistance when applied over ametallic substrate such as, for example, when applied as an automotivevehicle primer coat over bare sheet steel. While not wishing to bound bytheory, the exceptional corrosion inhibition provided by the inventionis believed to stem, in part, from the presence of C-N bonds both in theurea moieties of the chain-extension reaction product and in theureylene crosslinking moieties. In addition, the absence of esterlinkages in the cured coating is believed to improve corrosionresistance. Ester linkages are known to be attacked by hydroxide, aproduct of the metal corrosion process. In contrast, the carbon-nitrogenbonds of the cured coating of the invention are believed to be highlyalkali resistant and thus highly resistant to degradation processesinvolving hydrolysis by cathodicly generated hydroxide, includinghydroxide generated by the corrosion of a metal substrate. Accordingly,the coating composition of the invention provide improved corrosionresistance in comparison to known coating compositions, for example,those comprising ester or even urethane linkages. Thus, for example,automotive vehicle primers comprising the coating composition of theinvention and having a calculated volatile organic content of 350-390g/l, have been discovered to provide corrosion resistance far superiorto that provided by typical low solids, e.g., greater than 500 g/lvolatile organic content, epoxy ester-based primers.

As presently understood, chain-extension reaction by each of the twoisocyanate functionality of a diblocked diisocyanate diurea oligomer,which are de-blocked during cure of the coating composition, providessubstantially linear chain-extension, in situ, on the surface of thesubstrate. De-blocked isocyanate functionality not undergoingchain-extension reaction is available for crosslinking reaction withactive N-hydrogen at the urea functionality of the polyurea polymerforming by chain-extension reaction during curing. Accordingly, it ispreferred that the blocked isocyanate groups and two of the epoxy of thepolyepoxide can be an end group. Reactions between such epoxy end groupsand de-blocked isocyanate end groups are believed to provide mostefficient chain-extension during cure.

EXAMPLE I Preparation of Half-blocked Diisocyanate

Ethanol, 9.07 g, was added to isophorone diisocyanate, 43.6 g, in methylamyl ketone, 13.2 g with 0.2 g dibutyl tin dilaurate. The mixture washeated at 60°-80° C. for 2 hours. The resulting half-blocked aliphaticdiisocyanate was cooled to room temperature and stored. The product wascharacterized by its infrared spectrum (after solvent evaporation) whichshowed a reduction of the isocyanate absorption at 2250 cm⁻¹ and thepresence of an intense carbonyl absorption at 1710 cm⁻¹.

EXAMPLE II Preparation of Diblocked Diisocyanate Diurea Oligomer

The diblocked diisocyanate diurea oligomer is prepared by adding 37. gof the alcohol half-blocked aliphatic diisocyanate prepared in Example Ito 5.8 g of 1,6-hexanediamine in 40. g of Hexyl Cellosolve Acetate. Therate of addition is controlled to maintain a gentle reflux. The reactionis stirred after completion of the addition for approximately 2 hours at60° C. and then cooled to room temperature and stored. Infraredspectroscopy shows no absorption for isocyanate by the product.

EXAMPLE III Preparation of Coating Composition of the Invention

An automotive vehicle primer comprising a coating composition accordingto the invention is prepared by mixing a pigment package and a resinpackage, the following:

    ______________________________________                                        Pigment Package                                                               ______________________________________                                        Silica           23.6 g                                                       Barytes          21.4 g                                                       Carbon black      0.3 g                                                       Titanium dioxide  5.8 g                                                       ______________________________________                                    

    ______________________________________                                        Resin Package                                                                 ______________________________________                                        Diblocked diisocyanate                                                                           30. g                                                      diurea oligomer.sup.1                                                         Epon 828.sup.2     20. g                                                      Propasol B.sup.3   36. g                                                      Dibutyl tin dilaurate                                                                             .4 g                                                      ______________________________________                                         .sup.1 The oligomer is prepared as described in Example II.                   .sup.2 Epon 828 is a trademark of Shell Chemical Company, Houston, Texas,     for Bishenol A epichlorohydrin epoxy resin.                                   .sup.3 Propasol B is a trademark of Union Carbide Corporation, New York,      New York for organic solvent (nbutoxypropanol).                          

The pigment package is thoroughly dispersed into the binder package. Theresulting composition is ready for use as an automotive primer. It has acalculated volatile organic content of about 370 g/l, and a viscosity ofless than 40 sec., #4 Ford Cup, at 27° C.

EXAMPLE IV Preparation of Cured Coating

The coating composition of Example III is filtered, sprayed on Parkercold rolled bare unpolished steel panels and baked at 180° C. for 30minutes. The resulting cured coating has excellent solvent and humidityresistance and shows little or no adhesion loss after 24 hours saltspray exposure according to ASTM test method B117 using a Singleton SCCHCorrosion test cabinet operated at 35°±2° C.

EXAMPLE V A. Preparation of Half-blocked Diisocyanate

Alcohol half-blocked aliphatic diisocyanate is prepared according to theprocedure of Example I.

B. Preparation of Diblocked Diisocyanate Diurea Oligomer

The diblocked diisocyanate diurea oligomer is prepared by adding 37.0 gof the alcohol half-blocked aliphatic diisocyanate prepared in Part A to99. g of 4,4'-methylenedianiline in 60. g methyl ethyl ketone. The ratioof addition is controlled to maintain a gentle reflux. The reaction isstirred after completion of the addition for approximately 2 hours at60° C. and then cooled to room temperature and stored. Infraredspectroscopy shows no absorption for isocyanate by the product.

EXAMPLE VI Preparation of Coating Composition of the Invention

An automotive vehicle primer comprising a coating composition of theinvention is prepared by mixing a pigment package and a resin package asfollows:

    ______________________________________                                        Pigment Package                                                               ______________________________________                                        Silica           23.6 g                                                       Barytes          21.4 g                                                       Carbon black      0.3 g                                                       Titanium dioxide  5.8 g                                                       ______________________________________                                    

    ______________________________________                                        Resin Package                                                                 ______________________________________                                        Diblocked diisocyanate                                                                           30. g                                                      diurea oligomer.sup.1                                                         Epon 1001.sup.2    40. g                                                      Propasol B.sup.3   60. g                                                      Dibutyl tin dilaurate                                                                             .4 g                                                      ______________________________________                                         .sup.1 Prepared as described in Example V.                                    .sup.2 Epon 1001 is a trademark of Shell Chemical Company, Houston, Texas     for a Bisphenol A epichlorohydrin epoxy resin.                                .sup.3 Propasol B is a trademark of Union Carbide Corporation, New York,      New York for organic solvent (nbutoxypropanol).                          

The pigment package is thoroughly dispersed into the binder package. Theresulting composition is ready for use as an automotive primer.

EXAMPLE VII Preparation of Cured Coating

The coating composition of Example VI is sprayed on bare, unpolishedsteel panels and baked at 180° C. for 30 minutes. The resulting curedcoating has excellent solvent and humidity resistance and shows littleor no adhesion loss after 24 hours salt spray exposure according to ASTMtest method B117 using a Singleton SCCH Corrosion test cabinet operatedat 35°±2° C.

Particular embodiments of the present invention described above areillustrative only and do not limit the scope of the invention. It willbe apparent to the skilled of the art in view of the foregoingdisclosure that modifications and substitutions can be made withoutdeparting from the scope of the invention.

I claim:
 1. A solvent based resin composition comprising: A. achain-extendable crosslinkable diblocked diisocyanate diurea oligomer ofnumber average molecular weight about 300 to about 5000 comprising thereaction product of diamine of molecular weight about 50 to about 700,with half-blocked organic diisocyanate of molecular weight about 120 toabout 2000 in molar ratio of about 1:2, respectively, said half-blockedorganic diisocyanate comprising the reaction product of a monofunctionalblocking agent with organic diisocyanate in molar ratio of about 1:1,said oligomer having de-blocking temperature of about 120° C. to about250° C.B. polyepoxide bearing about 2 to about 10 epoxide groups andhaving molecular weight of about 100 to about 1000, in a weight ratio tosaid oligomer of about 1:1 to about 1:10; and C. organic solvent.
 2. Theresin composition of claim 1 wherein said oligomer has a number averagemolecular weight of about 300 to about
 1500. 3. The resin composition ofclaim 1 wherein said oligomer has a deblocking temperature of about 150°C. to about 220° C.
 4. The resin composition of claim 3 wherein saiddiamine comprises alkylenediamine wherein the alkylene moiety isstraight or branched chain and has about 2 to about 20 carbons.
 5. Theresin composition of claim 4 wherein said diamine bears two terminalamine groups.
 6. The resin composition of claim 5 wherein said diamineis selected from the group consisting of 1,6-hexanediamine,1,5-pentanediamine, 1,4-butanediamine and a mixture of any of them. 7.The resin composition of claim 3 wherein said diamine comprisescycloaliphatic diamines of about 4 to about 20 carbons.
 8. The resincomposition of claim 7 wherein said diamine is isophorone diamine. 9.The resin composition of claim 1 wherein said diamine comprises aromaticdiamine wherein each amine is substituted on the same benzene ring or ondifferent benzene rings linked through a covalent bond or through one ormore carbons of a one to seven carbon aliphatic moiety.
 10. The resincomposition of claim 9 wherein said diamine is selected from the groupconsisting of toluene diamine, 4,4-methylenedianiline and a mixture ofthem.
 11. The resin composition of claim 1 wherein said monofunctionalblocking agent is selected from the group consisting of primary alcohol,secondary alcohol, amide, phenol, ketoxime and a mixture of any of them.12. The resin composition of claim 1 wherein said monofunctionalblocking agent is alcohol of one to about eight carbons.
 13. The resincomposition of claim 1 wherein said monofunctional blocking agent isethanol.
 14. The resin composition of claim 1 wherein said polyepoxidehas 2 to about 4 epoxide groups.
 15. The resin composition of claim 1wherein said polyepoxide bears two terminal epoxide groups.
 16. Theresin composition of claim 1 wherein said polyepoxide comprisesdiepoxide.
 17. The resin composition of claim 16 wherein said diepoxideis selected from the group consisting of 1,4-butandiol digylcidyl ether,4-vinylcyclohexenedioxide, Bisphenol A epichlorohydrin epoxy resin,cycloaliphatic diepoxy resin, hydantoin epoxy resin, epoxy novalak resinand a mixture of any of them.
 18. The resin composition of claim 1wherein said oligomer and said polyepoxide are present in molarequivalent ratio of about 1:1 to about 2:1.
 19. The resin composition ofclaim 1 wherein said solvent is selected from the group consisting ofbutanol, methyl amyl ketone, methyl ethyl ketone and a mixture of any ofthem.
 20. A solvent based resin composition comprising:A. the reactionproduct of diamine selected from the group consisting of1,6-hexanediamine, 1,5-pentanediamine, 1,4-butanediamine, toluenediamine, methylenedianiline and a mixture of any of them, withhalf-blocked organic diisocyanate in molar ratio of about 1:2,respectively, said half-blocked diisocyanate comprising the reactionproduct of monofunctional blocking agent selected from the groupconsisting of phenol, amide, primary alcohol, secondary alcohol,ketoxime and a mixture of any of them, with an approximately equal molaramount of organic diisocyanate of molecular weight less than about 250,said oligomer having a number average molecular weight of about 300 toabout 1500 and a de-blocking temperature of about 150° C. to about 220°C.; B. polyepoxide of molecular weight about 300 to about 700, bearing 2to about 4 epoxide groups, in weight ratio to said oligomer of about 1:1to about 1:2, respectively; and C. organic solvent in amount sufficientto provide a volatile organic content (calculated) of less than about400 g/l.
 21. The resin composition of claim 20 wherein said organicdiisocyanate is selected from the group consisting of toluenediisocyanate, phenylene diisocyanate, isophorone diisocyanatediisocyanatoalkane wherein the alkylene moiety has about 3 to about 10carbons and a mixture of any of them.
 22. The resin composition of claim20 wherein said polyepoxide is Bisphenol A epichlorohydrin epoxy resin.23. A method of making a corrosion, humidity and solvent resistantcoating, in situ, on the surface of a substrate, which method comprisesapplying to said surface a solvent based resin composition comprising:A.a chain-extendable crosslinkable diblocked diisocyanate diurea oligomerof number average molecular weight about 300 to about 5000 comprisingthe reaction product of diamine of molecular weight about 50 to about700, with half-blocked organic diisocyanate of molecular weight about120 to about 2000 in molar ratio of about 1:2, respectively, saidhalf-blocked organic diisocyanate comprising the reaction product of amonofunctional blocking agent with organic diisocyanate in molar ratioof about 1:1, said oligomer having de-blocking temperature of about 120°C. to about 250° C. B. polyepoxide of molecular weight about 100 toabout 1000, in weight ratio to said oligomer of about 1:1 to about 1:10;and C. organic solvent; andheating said composition at about 120° C. toabout 250° C. for a time sufficient to cure said composition.
 24. Acorrosion, humidity and solvent resistant coating made, in situ, on thesurface of a substrate according to the method of claim 23.